Typically, an active material is used to produce useful work or a structural shape alteration through changes in length. Changing the length to produce a useful effect operates if the application and the properties of the active material (e.g., the specific force, stress, pressure, and strain capability) are well-matched. However, in most applications there is not a good match between a particular application and corresponding active material properties. A strategy for modifying the force/displacement output of the active material is to use lever-type mechanisms or other structural means to amplify or reduce stroke, with a concomitant change in output force. Such mechanisms can be complex and prone to failure.
Another common issue with active materials is the issue of power-off hold. In order to retain the desired shape change, the active material must be continuously powered. Because the active material must be continuously powered, the material is unable to retain its shape when powered off. This is particularly problematic where the active material is activated intermittently or remotely.
A few patents have been issued that disclose variable volume devices. U.S. Pat. No. 6,405,532, issued to M. Shahinpoor and K. J. Kim (hereinafter “the Shaninpoor patent”), describes a bladder-type actuator where the force transfer is accomplished through the uptake and release of hydrogen gas by a metal hydride into an inflatable bladder or variable volume device. While a metal hydride may be considered as a volume within which the gas is compressed, the Shahinpoor patent does not disclose using active materials or other mechanisms to accomplish similar goals, or to accomplish power-off hold of the actuator. Additionally, the Shahinpoor patent does not disclose possibilities for distributed actuation of larger or more complex structures.
U.S. Pat. No. 6,349,903, issued to J. H. Caton et. al; (hereinafter “the Caton patent”) describes flexible reinforced elastomeric control surfaces that may be actuated through the use of pneumatically actuated bladders or shape memory alloy wires embedded in the elastomer. As with the Shahinpoor patent, the Caton patent does not disclose the use of active materials to accomplish changes in volume of the bladders, or methods by which the motion of the control surfaces may be fixed without a continued application of power.
Thus, a continuing need exists for a volume-conversion technique that constricts a volume using an active material, and uses a force transfer medium that might be reversibly or irreversibly frozen in place.